In firefighting, water has certain disadvantages which reduce its efficacy in extinguishing fires. The primary effect of water on fire is cooling, thereby reducing the ability of the fuel to burn, and displacement of oxygen necessary for the combustion of fuel. Unfortunately, water has a relatively high surface tension with attendant poor wetting properties for many surfaces. Water also has a low viscosity and flows well at any temperature between its freezing and boiling points. When water is brought into contact with very hot surfaces it has a tendency to bead and roll off the surface. This phenomenon is caused by the formation of a layer of steam between the surface and the water which acts to insulate the water from direct contact with the hot surface, reducing the ability of the water to absorb heat from the surface or displace oxygen.
Additives can be introduced to water used for firefighting to reduce the inherent disadvantages of water as a fire extinguishing agent. Surfactants can be added to water to improve its wetting properties. Gelling agents can be added to water to form a gel for use as a firefighting agent. Such gels, however, lose cohesion and viscosity on contact with hot surfaces. Foaming agents combined with water can be quite effective in smothering fires under special conditions. However, foams cannot be used in large fires since high winds created by major fires dissipate the foam or prevent its accurate application. In addition, in the case of chemical fires, burning chemicals frequently cause the collapse of the foam and consequent loss of most of its fire extinguishing properties.